An electrochemical measuring cell of the kind referred to above is disclosed in U.S. Pat. No. 4,051,006. In this known measuring cell, a cylindrical round body having two end faces is made of polytetrafluoroethylene. Each of the two end faces is provided with a ring-shaped cover and a clamping ring. The cover and clamping ring at each end clamp a porous membrane which is permeable for gaseous constituents but is impermeable for the electrolyte disposed in the inner space of the housing. The gaseous constituent of the air to be detected communicates with the membrane via a diffusion path and, in this way, allows the gas to be detected to enter into the inner space filled with the electrolyte. The inner space likewise accommodates the measuring electrode in direct proximity to the diffusion membrane. The electrolyte is located in a hydrophilic fill body or filler made of glass wool and is in liquid contact between the measuring electrode and the counter electrode. The counter electrode is disposed within the filler and is completely surrounded by the electrolyte liquid.
The membrane clamped by the ring is likewise permeable to the gaseous constituents; however, the membrane is impermeable for the electrolyte. The membrane functions to output the gaseous components to the ambient which are formed during the electrochemical reaction.
A problem occurring with the known electrochemical measuring cell has to do with equalizing the pressure between the chamber or the interior space of the housing which holds the electrolyte and the ambient. The electrolyte chamber is generally only partially filled because the electrolyte volume changes with ambient conditions (moisture, temperature). For this reason, a gas bubble is normally present in the interior space of the sensor. If the ambient pressure drops, for example, during transport in an aircraft, then an overpressure occurs in the sensor which can only be reduced without difficulty if at least one of the gas-permeable membranes is not covered by the electrolyte. This is possible in the known measuring cell only under favorable conditions with a perpendicular orientation and without an electrolyte surplus. However, if the membranes are covered by the electrolyte, the electrolyte is pressed through the membrane or possibly pressed through capillary gaps during pressure equalization. This can lead to an endangerment of the user, damage to the measuring device and to irreversible changes of the membrane.
The change of the measuring electrode membrane, in turn, causes the measured values to be incorrect and can lead to a complete loss of sensitivity. If the membrane withstands the liquid pressure, then it can nonetheless be deformed so that the diffusion characteristics and therefore the measuring quality change.